Lateral acoustic waves, also referred to as Lamb waves, may propagate through portions of an active region of a bulk acoustic wave resonator due to finite lateral dimensions of the BAW resonator structure. This may result in part of the energy contained in a fundamental thickness mode (i.e., a vertically resonating mode) leaking to lateral modes (i.e., a horizontally resonating mode), which results in a degradation of a quality factor of the BAW resonator. Lateral acoustic waves may become evident in electrical behavior of the BAW resonator in the form of spurious resonances leading to strong ripples in the bandpass frequencies.
Performance of a BAW resonator may be improved by creating a region with specific boundary conditions in which lateral acoustic waves cannot propagate. For some resonators, this may be done by including a thickened edge load, known as a border ring (BO), in a perimeter of an active region of the BAW resonator. Presence of the thickened edge enables the mismatch between the active and inactive regions to be avoided, providing a smooth transition of propagating waves in the active region to evanescent waves in the inactive region.